Particles are better then "brush weather" because:
So, tfrepl, how do you add particle weather? Is it hard?
Let me answer that question with an answer:
1. Create an info_particle_system about 512 units above the floor/structure/building (experiment with the height for better results).
2. Edit the following in your info_particle_system:
Particle System Name: env_rain_001 (for rain) or env_snow_light_001 (for snow).
Start Active?: Yes
Flag as Weather?: Yes
Notes: Flag as Weather is used to remove the particles if the user has chosen to remove weather.
Particle System Name: there could be other options (showed bellow).
The more info_particle_system's you add the more dense the weather will be.
You might want to add func_precipitation, to fill in the gaps.
3. You're done. Here's a comparsion and a list of weather effects.
Zoom in for better effect.
Some of the weather particles:
env_rain_001 env_rain_001_mist env_rain_001_mist_256 env_rain_001_streaks env_rain_001_streaks_256 env_rain_002_256 env_rain_gutterdrip env_rain_guttersplash env_rain_ripples env_rain_splashring env_rain_splashripple env_snow_light_001 snow_steppuff01 snow_steppuff_mist water_burning_steam water_playerdive_bubbles water_splash01 water_splash01_bullet water_splash01_cluster water_splash01_column water_splash01_droplets water_splash01_playerripple wateR_splash01_ripple waterfall_bottommist waterfall_bottomsplash waterfall_bottomwaves waterfall_mist waterfall_rocksplash waterfall_rocksplash_2 waterfall_sides
Hope this helped :) .